Codecs (encoders/decoders) are used in wireless communication systems to compress speech/voice signals in order to utilize efficiently the expensive bandwidth resources both in the radio interface and in the transmission networks. At the same time, transcoding of voice signals may significantly degrade signal quality and therefore unnecessary transcoding should be avoided.
In a conventional call configuration between wireless user terminals the speech signal is first encoded in the originating user terminal, sent over the radio interface, converted to PCM (Pulse Code Modulated) coded signal (e.g. A-law or μ-law ITU-T Recommendation G.711) in a local transcoder, carried over a fixed transmission network, transcoded again in a distant transcoder, sent over a distant radio interface and finally decoded in the terminating user terminal. In this configuration, the two speech codecs are in so called tandem operation, which may cause degradation in speech quality due to multiple transcodings.
This conventional configuration is illustrated in FIG. 1, which shows a system comprising two interconnected networks, network A 101 and network B 102. Both networks comprise a transcoding function 105 and 106. Further user terminals 103 and 104 are connected to the networks A and B respectively. Messaging line 107 illustrates the conventional encoding/decoding, which takes place in different parts of the system.
For avoiding the double coding and decoding of voice signals by intermediate network elements, methods called Tandem Free Operation (TFO) and Transcoder Free Operation (TrFO) have been introduced. The principle in TFO and TrFO is to transmit compressed signal, which is encoded in a user terminal, as such in a fixed transmission network, whereby transcoding in the transmission network may be avoided. When the originating user terminal and terminating user terminal are using/can use the same speech codec or codec mode, these techniques make it possible to transparently transmit the speech frames received from the originating user terminal to the terminating user terminal without activating the transcoding functions in the originating and terminating networks. This is illustrated in FIG. 1 with messaging line 108. The details of TFO and TrFO are slightly different, but the principle is the same in both methods.
TFO and TrFO allow changing of codec mode used in the associated user terminals, if the situation in the radio interface of one or the other end requires that. It however may take end-to-end round trip amount of time before the change in the codec mode is effected. Thus there may be considerable delay, which may have an effect on signal/speech quality. For example fast degradation of radio interface may cause bad frames and thereby degradation in speech quality until codec mode will be changed to one that is suitable for the degraded radio interface. The delay in effecting a codec mode change in connection with TrFO and TFO is illustrated in messaging diagrams of FIGS. 2 and 3 respectively.
In FIG. 2, UE_B (User Equipment) first transmits with codec mode X 2-1 to UE_A. Then UE_A sends to RNC_A (Radio Network Controller) a measurement report 2-2 indicating a situation in which there is a need to change codec mode in downlink radio connection of UE_A. (This measurement report may have been requested by the RNC_A.) In phase 2-3, RNC_A performs rate control on the basis of the measurement report 2-2. RNC_A sends rate control information 2-4 to RNC_B, which then performs measurement of uplink radio connection of UE_B in phase 2-5. In phase 2-6 RNC_B performs rate control on the basis of the rate control information 2-4 received from RNC_A and measurements performed in phase 2-5. RNC_B sends the maximum rate found out as a result of rate control 2-7 to UE_B. Then UE_B is able to send speech frames to UE_A with codec mode Y 2-8.
Arrow 200 illustrates the delay in effecting the codec mode change in this arrangement. This delay may be for example 450 ms or even more in TrFO between 3G (3rd generation mobile phone) terminals.
In FIG. 3, MS_B (Mobile Station) initially sends speech frames with 12.2 kbits/s AMR (Adaptive Multi Rate) to MS_A (not shown in figure). MS_A detects need to lower the codec rate in downlink radio connection of MS_A to 7.4 kbit/s and sends a downlink codec mode request (DL CMR) 3-1 to a transcoder_A/MGW_A (Media Gateway). A TFO functionality in the transcoder_A/MGW_A forwards the DL CMR 3-2 to a TFO functionality in a transcoder_B/MGW_B, which forwards the DL CMR 3-3 to a decoder in MS_B. The decoder in MS_B forwards the DL CMR 3-4 to an encoder in MS_B. After receiving the DL CMR 3-4 the encoder in MS_B is able to change the coded mode used in sending speech frames to MS_A to 7.4 kbit/s AMR 3-5, which is sent to an decoder in the transcoder_B/MGW_B, forwarded to an encoder in the transcoder_A/MGW_A 3-6 and to a decoder in MS_A 3-7.
Arrow 300 illustrates the delay in effecting the codec mode change in this arrangement. This delay may be for example over 1 s or even more in TFO between GSM terminals.
Hence, there is a need to further develop speech coding arrangements in communication networks.